wrapper: use trace2 counters to collect fsync stats

As mentioned in the thread starting at [1], trace2 counters should be
used to count events instead of ad-hoc static variables.

Convert the two fsync static variables to trace2 counters, reducing the
coupling between wrapper.c and the trace2 subsystem. Adjust t/t5351 to
match the trace2 counter output format.

The counters are not per-thread because the ones being replaced also
were not.

[1] https://lore.kernel.org/git/20230627195251.1973421-2-calvinwan@google.com/

Signed-off-by: Beat Bolli <dev+git@drbeat.li>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

trace2.h

@@ -552,6 +552,10 @@ enum trace2_counter_id {
 	TRACE2_COUNTER_ID_TEST1 = 0, /* emits summary event only */
 	TRACE2_COUNTER_ID_TEST2,     /* emits summary and thread events */
 
+	/* counts number of fsyncs */
+	TRACE2_COUNTER_ID_FSYNC_WRITEOUT_ONLY,
+	TRACE2_COUNTER_ID_FSYNC_HARDWARE_FLUSH,
+
 	/* Add additional counter definitions before here. */
 	TRACE2_NUMBER_OF_COUNTERS
 };